An analytical tool and an analytical apparatus are used in combination to analyze a particular component contained in a biochemical sample. For instance, to measure a glucose level (blood glucose level) contained in blood, a biosensor and a blood glucose level measuring apparatus are used in combination.
FIGS. 9-11 show a conventional biosensor. The biosensor 8 shown in the figures measures the blood glucose level by an electrochemical method and includes a substrate 80 to which a cover 82 is bonded via a spacer 81. The spacer 81 is formed with a slit 83, which defines a capillary 84 between the substrate 80 and the cover 82. A reagent layer 85 is provided in the capillary 84, so that a reaction field for reaction between blood and a reagent contained in the reagent layer 85 is provided when blood is introduced into the capillary. The substrate 80 is provided with a working electrode 86 and a counter electrode 87. The working electrode 86 and the counter electrode 87 are used for applying a voltage to the reaction field and measuring the response current obtained at the time.
To use the biosensor 8, such a blood glucose level measuring apparatus as shown in FIG. 12 is used. The blood glucose level measuring apparatus 9 shown in the figure includes connectors 90, 91 for coming into contact with the working electrode 86 or the counter electrode 87 and applying a voltage across the working electrode 86 and the counter electrode 87. The connector 90, which is to come into contact with the working electrode 86, is connected to a current-voltage conversion circuit 92 and an A/D conversion circuit 93. The connector 91, which is to come into contact with the counter electrode 87, is connected to ground.
The blood glucose level measuring apparatus 9 determines that blood is introduced into the capillary 84 when electrical connection between the working electrode 86 and the counter electrode 87 is detected and computes the blood glucose level based on the response current measured after the lapse of a predetermined period from the detection of the blood introduction.
As noted above, the blood glucose level measuring apparatus 9 determines that blood is introduced into the capillary 84 at the time point when electrical connection between the working electrode 86 and the counter electrode 87 is detected. Specifically, it is determined that blood is introduced into the capillary 84 when the response current exceeding a predetermined threshold is measured. Thus, in the blood glucose level measuring apparatus 9, the blood glucose level measurement is performed even when the amount of blood introduced into the capillary 84 is not sufficient for accurate measurement, though the blood is supplied. Thus, when the obtained blood glucose level is low, there is a possibility that the low value is caused by insufficient blood amount.
FIGS. 13 and 14 show another biosensor (see Patent Document 1, for example). In FIGS. 13 and 14, the elements which are identical or similar to those of the biosensor 8 shown in FIGS. 9-11 are designated by the same reference signs as those used for the biosensor 8.
The biosensor 8′ further includes a detection electrode 88 in addition to the working electrode 86 and the counter electrode 87. The detection electrode 88 is utilized for determining whether or not blood is introduced into the capillary 84, and the portion of the detection electrode which is to come into contact with blood is provided on a downstream side in the blood flow direction (deeper side) in the capillary 84. Thus, the fact that the blood has reached the detection-electrode 88 means that the blood has reached both of the working electrode 86 and the counter electrode 87 and thus means that a sufficient amount of blood is introduced into the capillary 84.
To use the biosensor 8′, such a blood glucose level measuring apparatus as shown in FIG. 15 is used. In FIG. 15, the elements which are identical or similar to those of the blood glucose level measuring apparatus 9 shown in FIG. 12 are designated by the same reference signs as those used for the blood glucose level measuring apparatus 9.
The blood glucose level measuring apparatus 9′ further includes a connector 94 for coming into contact with the detection electrode 88 in addition to the connectors 90 and 91. The connector 94 is connected to ground. Specifically, by opening or closing the switch 96 by a CPU 95, the connector 91 is connected to ground or not connected to ground.
The blood glucose level measuring apparatus 9′ determines that blood is introduced into the capillary 84 when electrical connection between the detection electrode 88 and the working electrode 86 is detected, with the switch 96 closed. Further, with the switch 96 opened, the blood glucose level measuring apparatus 9′ applies a voltage across the working electrode 86 and the counter electrode 87 via connectors 90, 91 and measures the response current obtained at the time. The blood glucose level is computed based on the response current measured after the lapse of a predetermined period from the detection of the blood introduction.
As noted above, the biosensor 8′ includes a detection electrode 88, and the portion of the detection electrode 88 which is to come into contact with blood is provided on a deeper side relative to the portion of the working electrode 86 and the counter electrode 87 which is to come into contact with blood. Thus, the determination that blood is introduced into the capillary 84 is made only when the amount of the blood introduced into the capillary 84 is sufficient.
However, the provision of the detection electrode 88 for detecting blood introduction into the capillary 84 in addition to the working electrode 86 and the counter electrode 87 increases the manufacturing cost of the biosensor 8′.
Further, the blood glucose level measuring apparatus 9′ used for the biosensor 8′ requires the additional connector 94 for the detection electrode 88. Moreover, it is also necessary to provide the switch 96 for selecting the state in which the connector 94 is connected to ground or the state in which the connector is not connected to ground and to control the on/off operation of the switch 96. Thus, as compared with the blood glucose level measuring apparatus 9 (see FIG. 12) used for the biosensor 8 (see FIGS. 9-12) which does not include a detection electrode 88, this blood glucose level measuring apparatus has a complicated structure and needs various control. Thus, the manufacturing cost of the blood glucose level measuring apparatus 9′ is higher than that of the blood glucose level measuring apparatus 9 (see FIG. 12).
Patent Document 1: JP-A-2001-208715